The present invention relates to a secondary air supply system for an engine of a motorcycle.
There are known some types of engines including a structure in which, in order to clean or purify an exhaust gas discharged from a combustion chamber of the engine, a secondary air is supplied into an exhaust passage thereby to burn the exhaust gas to promote an oxidation of the exhaust gas.
For such engine structure, it is generally known as a secondary air supply method to use a reed valve which is opened by a negative pressure of a pressure wave of the exhaust gas, and in such structure, an air-cut valve for restricting the flow of the secondary air is connected so as to prevent an excessive air from entering into the exhaust gas passage.
In a case where the secondary air supply system of the structure mentioned above is equipped to an engine of a motorcycle, there is known a structure in which, for example, a reed valve and an air-cut valve are arranged independently or disposed to a frame disposed above the engine, or the reed valve and the air-cut valve are integrally arranged thereto.
However, in the arrangement in which the reed valve and the air-cut valve are disposed independently, the number of pipes, ducts or the like will increase and, hence, the arrangement or structure thereof will become complicated, which will results in cost increasing.
Moreover, in the arrangement in which the reed valve and the air-cut valve are disposed to the frame above the engine, a long pipe, duct or the like is required for connecting the reed valve and an exhaust valve. Further, since the reed valve and the air-cut valve must be located directly below a fuel tank, there is a fear of limiting a shape or inner volume of the fuel tank.
Furthermore, because the secondary air supply system must be disposed after the engine has been mounted to the frame, this assembling working must be performed in a narrow space, thus being inconvenient.
Still furthermore, in a case of a multi-cylinder engine, there is a fear that lengths of the pipes arranged between the reed valve and the respective exhaust pipes may not be made equal to each other in a certain location of the reed valve and the air-cut valve, and in such case, the exhaust gas cleaning effects may become different for the respective cylinders.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a secondary air supply system for an engine of a motorcycle in which pipes, ducts or the like is made short in arrangement and assembling working can be easily and simply done.
This and other objects can be achieved according to the present invention by providing a secondary air supply system for an engine of a motorcycle, which is provided with a secondary air valve composed of integrated reed valve and air-cut valve and exhaust pipes are connected to exhaust ports formed at a front portion of the engine, wherein the secondary air valve is arranged at substantially a central portion in front of the engine, below the exhaust ports and in a space between the exhaust pipes and the engine, and secondary air pipes extending from the secondary air valve are connected to secondary air ports formed to an upper portion of the exhaust ports.
In a preferred embodiment of the secondary air supply system, the secondary air pipes once extend upward from the exhaust ports, then extend downward between the exhaust pipes along a front wall section of the engine and are then connected to the secondary air valve through heat resistant hoses, respectively.
The engine is provided with an even number of cylinders arranged in parallel to each other and the secondary air pipes are arranged between the parallel cylinders. In a desired example, the engine is a four-stroke-cycle engine provided with four cylinders respective two of which are arranged in parallel to each other.
The secondary air valve has a valve body in which an integrated structure of a reed valve and an air-cut valve is housed and the secondary air valve is mounted to a crank case of the engine through a bracket.
According to the structure or arrangement of the secondary air supply system for a motorcycle engine mentioned above, the pipe or duct arrangement can be made simple with short pipes or like, and, hence, the assembling working thereof can be also made simple and easy. The heat resistant hoses are used for the connection of the secondary air pipes to the secondary air valve, so that the heat resistant property can be improved.
In the parallel arrangement of the even number of cylinders, the pipe length can be made substantially equal, so that the exhaust gas cleaning function of the respective cylinders can be effectively made uniform.
The nature and further characteristic features of the present invention will be made further clear from the following descriptions made with reference to the accompanying drawings.